Current communication technology assigns or licenses a channel or a set of channels, identified by predefined bandwidths, to a particular type of transmitting device. The transmitting device has an "occupied bandwidth" which is the bandwidth in which a majority of the power or energy is limited. The occupied bandwidth must be less than the assigned or licensed bandwidth so that the operation of one device does not interfere with the operation of a device assigned a neighboring bandwidth. Under these general constraints, it is desirable to maximize the occupied bandwidth such that it closely approximates the assigned bandwidth without exceeding the boundaries of the assigned bandwidth.
In frequency modulated "FM systems", the occupied bandwidth of the transmitter is a function of both the modulating signal bandwidth (or symbol rate in digital FM) and the deviation. The deviation is typically a difficult to control parameter leading to one of several compromise situations. The first compromise situation is the use of precision components to decrease the variability of the deviation. Such precision components are rather expensive which leads to a rather substantial increase in the overall cost of the transmitting device. The second compromise situation is the use of labor intensive factory tuning. The factory tuning of a transmitting unit decreases the variability of the deviation but also adds to the cost as well as decreasing product reliability. Yet a third compromise situation is to use neither precision components nor factory tuning. In this third situation, the deviation tolerance is poor, meaning that the unit with the worst case maximum deviation must have an occupied bandwidth that is still within the assigned bandwidth. In this third situation, the average device has a deviation, and corresponding occupied bandwidth which is dramatically underutilized.
Prior art devices have used a synthesizer to generate frequencies that are phase locked to a given reference frequency. Switching from one output frequency to another is accomplished by changing the loop frequency divider coupled to the feedback and then allowing the loop feedback control system to "slowly" take the voltage controlled oscillator from the old frequency to the new frequency.
Prior art devices have used a step voltage to "presteer" the voltage controlled oscillator to the new frequency in a short period of time. Some prior art devices also provide a feedback mechanism to keep the step voltage at a nominal value. This type of feedback mechanism, in all of its varying embodiments, waits for a steady state condition to develop in the loop and then directly measures the voltage applied to the VCO relative to that which was "injected" for presteering purposes. Such a mechanism is relatively slow, since it must wait for steady state and is generally less accurate because of the small size of the voltage at the VCO and the error in directly measuring the voltage applied to the VCO.
U.S. Pat. No. 5,027,087 to Rottinghaus (hereinafter referred to as "Rottinghaus '087"), assigned to the assignee of record in the present application, discloses a feedback apparatus and method in the context of frequency presteering. The feedback technique disclosed in Rottinghaus '087 measures the loop transient response following the injection of a presteering voltage step. By examining the slope of the feedback transient, the size of the presteering voltage step can be determined to be, "too large", or "too small", thereby allowing correction of the presteering voltage in accordance with an algorithm. The device and method as shown in Rottinghaus '087 provides better results than other prior art devices and methods since the detection of the accuracy of the step voltage can be done immediately following the step without waiting for a steady state to develop in the loop. The device and method of Rottinghaus '087 generally is not subject to measurement errors of voltage gain or offset since the device and method is simply observing the loop response to the error in the presteering step. In other words, the function of the loop, switching between frequencies, is replaced by a presteering mechanism within the same loop which is made more accurate through a feedback technique.
Rottinghaus '087 also discloses a method for aligning the modulator. Once the presteering step is aligned for a given frequency transition, the slope of the VCO is known. The slope information can also be used to control the FM deviation of the modulator. This method for aligning the modulator works well, but only within a certain tolerance.
Another apparatus and method as shown in U.S. Pat. No. 5,207,491 to Rottinghaus (hereinafter referred to as "Rottinghaus '491"), which is assigned to the assignee of the present invention, attempts to improve upon the teachings of Rottinghaus '087. Rottinghaus '491 shows a presteering transient wave form which replaces the simple step voltage of Rottinghaus '087. Also disclosed in Rottinghaus '491 is a method of measuring the output of a phase detector. Two algorithmic embodiments are disclosed for post processing the measured phase detector output. The purpose of this post processing is to correct the transient response size and shape. In other words, while Rottinghaus '087 corrected the size of the transient response, Rottinghaus '491 attempts to correct both the size and shape.
While the Rottinghaus '087 and '491 devices and methods attempt to improve the use of an assigned bandwidth, these prior art references fall short of teaching devices and/or methods that optimize and efficiently use an assigned bandwidth. As such, it would be desirable to provide a device and method for optimizing the use of an assigned bandwidth with greater efficiency, precision and repeatability than prior art devices and methods .